What are the commonly used STL function objects in the C++ standard library?

C STL provides a variety of function objects that can be used to compare, sort and operate elements. Common function objects include less for ascending sorting, greater for descending sorting, equal_to for comparing equality, and bind2nd and mem_fn for binding function parameters. In practice, you can sort an array in descending order by using the greater function object, as follows: Using the sort() function, the greater function object will sort the elements in the specified range in descending order.

Commonly used STL function objects in the C standard library

The function object, namely Functor, is a function object that can be called and returned The result object. The C standard library provides many useful STL function objects for use in a variety of algorithms and operations.

The following are some commonly used function objects in C STL:

• less: Comparison function used to sort elements (ascending order ).
• greater: Comparison function used to sort elements (descending order).
• equal_to: Equal comparison function, used to check whether two elements are equal.
• not_equal_to: Not equal to comparison function, used to check whether two elements are not equal.
• greater_equal: Greater than or equal to comparison function, used to check whether an element is greater than or equal to another element.
• less_equal: Less than or equal to comparison function, used to check whether one element is less than or equal to another element.
• bind2nd: Binds a parameter of the function object and returns the bound function object.
• mem_fn: Creates a function object that creates a function object from a member function pointer.

Practical case:

Use the greater function object to sort the integer array in descending order:

#include <algorithm>
#include <vector>

using namespace std;

int main() {
  vector<int> myVector = {1, 3, 5, 2, 4};

  // 使用 greater 函数对象对 myVector 中的元素进行降序排序
  sort(myVector.begin(), myVector.end(), greater<int>());

  // 输出排序后的数组
  for (auto it = myVector.begin(); it != myVector.end(); ++it) {
    cout << *it << " ";
  }
  cout << endl;

  return 0;
}

Output:

5 4 3 2 1

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